1. Field of the Invention
The present invention relates to a wafer polishing apparatus and a wafer manufacturing method which are employed in a semiconductor production process for polishing surfaces of semiconductor wafers.
2. Description of the Related Art
Recently, with finer patterns resulting from an increased packing density of semiconductor devices, it has become more important to polish surfaces of semiconductor wafers as flat as possible during a production process so that, in particular, fine patterns of a multilayered structure can be easily and reliably formed. Under such situations, an attention has been focused on the chemical mechanical polishing (CMP) process which can provide a higher degree of flatness in polishing surface films.
The CMP process means a process for polishing and planarizing wafer surfaces in a chemical and mechanical manner with, e.g., a alkaline solution using SiO.sub.2 as an abrasive, a neutral solution using SeO.sub.2, or an acidic solution using Al.sub.2 O.sub.3. One example of wafer polishing apparatuses for implementing the CMP process is shown, by way of example, in FIG. 11.
Referring to FIG. 11, a wafer polishing apparatus 100 comprises a wafer holding head 101 for holding a wafer W to be polished, and a polishing pad 102 bonded to an overall upper surface of a platen 103 in the form of a disk. The wafer holding head 101 is provided in plural number and is mounted to the underside of a carousel 104 which serves as a head driving mechanism. Each wafer holding head 101 is rotatably supported by a spindle 111 and is rotated on the polishing pad 102 in planetary fashion. Incidentally, the center position of the platen 103 and the center about which the wafer holding heads 101 revolve may be offset from each other.
The platen 103 is horizontally arranged at the center of a base 105 and is rotatable about its axis by a platen driving mechanism provided within the base 105. Posts 107 are provided aside the base 105, and an upper mount plate 109 for supporting a carousel driving mechanism 110 is disposed between the posts 107. The carousel driving mechanism 110 has a function of rotating the carousel 104, provided below the mechanism 110, about its axis.
Abutment members 112 are disposed on the base 105 to project upward from the base 105, and spacing adjusting mechanisms 113 are provided on respective upper ends of the abutment members 112. Above the abutment members 112, engagement members 114 are disposed in one-to-one relation. The engagement members 114 are fixed to the upper mount plate 109 and are projected downward from it. The abutment members 112 and the engagement members 114 are brought into contact with each other while the spacing adjusting mechanisms 113 are operated to adjust a distance between the wafer holding heads 101 and the polishing pad 102 to a proper one. The wafers W held on the wafer holding heads 101 are thereby brought into contact with the surface of the polishing pad 102. The carousel 104 and the platen 103 are then rotated to polish the wafers W.
As shown in FIG. 12, the wafer holding heads 101 each comprise a head body 121 made up of a top plate 121a and a tubular peripheral wall 121b fixed to an outer periphery of the top plate 121a; a diaphragm 122 made of an elastic material such as rubber and stretched inside the head body 121; a disk-shaped carrier 123 fixed to a lower surface of the diaphragm 122; an annular retainer ring 124 disposed between an outer periphery of the carrier 123 and the peripheral wall 121b in concentric relation with small gaps left relative to them; and a pressure regulating mechanism 125 for regulating a pressure in a fluid chamber 126 defined between the head body 121 and the diaphragm 122.
The diaphragm 122 is held between the peripheral wall 121b and a diaphragm fixing ring 127, and it is fixed to the head body 121 by screws 127a tightened into the peripheral wall 121b from above the diaphragm fixing ring 127.
The carrier 123 is disposed on the lower side of the diaphragm 122, and a carrier fixing ring 128 is disposed on the upper side of the diaphragm 122 with the diaphragm 122 held between the carrier 123 and the carrier fixing ring 128. The carrier 123 is fixed to the diaphragm 122 by screws 128a tightened into the carrier 123 from above the carrier fixing ring 128.
The retainer ring 124 in the annular form is fitted to a circular groove defined between the peripheral wall 121b and the outer periphery of the carrier 123 such that the retainer ring 124 is positioned in concentric relation to the peripheral wall 121b and the carrier 123 while small gaps are left relative to both an inner surface of the peripheral wall 121b and an outer peripheral surface of the carrier 123. Further, a retainer-ring fixing ring 129 is disposed on the upper side of the diaphragm 122 with the diaphragm 122 held between the retainer ring 124 and the retainer-ring fixing ring 129. The retainer ring 124 is fixed to the diaphragm 122 by screws 129a tightened into the retainer ring 124 from above the retainer-ring fixing ring 129. The retainer ring 124 is of a two-piece structure comprising a metal-made upper portion into which the screws 129a are tightened, and a resin-made lower portion which is brought into contact with the polishing pad 102.
The wafer W is polished by the polishing apparatus including the above-described wafer holding head as follows. First, the wafer W is stuck to a wafer attraction sheet S, which is disposed on the underside of the carrier 123, so as to locate in imbedded fashion within the retainer ring 124. Then, the surface of the wafer W exposed to face downward is brought into contact with the polishing pad 102 bonded to the upper surface of the platen 103, and is polished under rotation of the wafer holding head while a slurry containing a polishing abrasive is supplied.
In the above operation, the carrier 123 and the retainer ring 124 are supported by a floating structure such that they can independently displace in the vertical direction with elastic deformation of the diaphragm 122. Therefore, pressing forces of the carrier 123 and the retainer ring 124 against the polishing pad 102 vary depending on the pressure in the fluid chamber 126 which is regulated by the pressure regulating mechanism 125.
A lower end of the retainer ring 124 projects downward of the carrier 123 and holds an outer periphery of the wafer W stuck to the lower surface of the carrier 123. This arrangement is intended not only to prevent the wafer W from dislodging from the carrier 123 during the polishing process, but also to prevent such a phenomenon that an outer peripheral area of the wafer W is polished in a larger amount than a central area thereof, by surrounding the wafer W with the retainer ring 124 and positioning a lower end surface of the retainer ring 124 at the same level as the lower surface (polished surface) of the wafer W.
In the above-described wafer polishing apparatus 100 in which the wafers W are polished by the polishing pad 102 with rotations of the wafer holding heads 101 and the platen 103, uniform polishing of the wafers W is achieved by satisfying such a condition that a relative speed, at which the polished surface of the wafer W and the polishing pad 102 rotate while contacting with each other, is uniform in a plane (hereinafter referred to as an "in-plane speed uniform condition). Assuming that the angular speed of the wafer holding head 101 is Rh, the angular speed of the platen 103 is Rp, and the angular speed of the carousel 104 is Rc, the in-plane speed uniform condition is expressed by the following relation and is satisfied when the following relation holds: EQU Rp=Rh+Rc (1)
By polishing the wafer W under the above condition, the wafer W is uniformly polished when an apparatus has an ideal construction. Also, under the above condition, both the wafer W and the wafer holding head 101 are not subjected to rotating forces. Accordingly, no torsion in the rotating direction occurs in the diaphragm which is one component of the head.
Depending on polishing environment such as accuracy of parts constituting the apparatus and in assembly of the parts or a material of the polishing pad 102, however, the polishing pad 102 is locally raised (hereinafter referred to as "waving deformation") in a portion T inward of the position at which the polishing pad 102 contacts the retainer ring 124, as shown in FIG. 13, and the wafer W is often not uniformly polished even under the in-plane speed uniform condition. The raised portion T of the polishing pad 102 polishes excessively an outer peripheral edge G of the wafer W, and raises a problem of impairing uniformity in polishing of the wafer W surface.
Conversely, depending on polishing environment, a central area of the wafer W is sometimes preferentially polished. In such a case, it is also attempted to polish the wafer W under a condition where the in-plane speed is not uniform. The condition where the in-plane speed is not uniform means such a speed condition that the above relation (1) does not hold. In general, if the in-plane speed is not uniform, an outer peripheral area of the wafer W tends to be preferentially polished. Making the in-plane speed not uniform is therefore effective when a polishing state of the wafer W under the in-plane speed uniform condition provides a lower polishing rate in the outer peripheral area of the wafer W than that in the central area thereof, i.e., when the central area of the wafer W tends to be preferentially polished.
As a result of polishing of the wafer W under the in-plane speed non-uniform condition based on the above consideration, however, a conventional head has not developed such a tendency that the outer peripheral area of the wafer W is preferentially polished. This is attributable to the fact that the diaphragm is twisted due to rotating forces generated at the non-uniform in-plane speed, and the polishing becomes unstable. Also, in a transition stage where the rotations of the respective components does not yet reach steady states in an initial period of the polishing, the in-plane speed non-uniform condition occurs necessarily and the polishing becomes unstable during the transition stage.